This invention relates to glass compositions suitable for use in radio-photoluminescence dosimetry and to improved radio-photoluminescent dosimeters characterized by containing said glass compositions as a source of latent fluorescent light radiation generated by exposure to gamma rays.
Glass dosimeters for measuring and recording ionizing radiation have been known for some 25 years. Those currently used are silver-bearing phosphate glass dosimeters which are similar to film dosimeters and are generally used for personnel dosimetry. These glass dosimeters, generally comprising two polished surfaces vertical to each other, are exposed to a ultra-violet light source after having been exposed to prior radiation, especially gamma radiation. The fluorescent radiation thus emitted is proportional to the gamma radiation which has been received, and is measured vertically to the direction of the UV-radiation incidence.
Dosimetry by means of silver phosphate glasses is based on the alternating effect of ionizing radiation at energy levels greater than about 10 keV on the silver ions in the glass. The nature of this alternating effect is still not completely clear, but the active centers formed by such radiation are believed to represent interference centers which, on energization with UV light at a frequency of about 365 nm, emit fluorescent light in the visible spectrum (the so-called radio-photoluminescence) which is then measured by means of a photoamplifier. The intensity of the emitted light is proportional to the gamma radiation dose received up to approximately 4.times.10.sup.3 Rad. Such silver-activated metaphosphate glasses should desirably show a low natural fluorescence upon UV light stimulation as well as a high sensitivity to radiation, especially gamma radiation.
The relative dose sensitivity of such dosimeter glasses depends on the energy level of the gamma radiation to which they are exposed, and remains substantially constant from about 0.2 MeV up to about 10 MeV. Below about 0.2 MeV, the dose indication rises to a maximum at approximately 50 KeV. A reduction in the effective order of magnitude of the chemical elements in the glass can be used to reduce their energy dependence, which may require compensation by encapsulation with perforated metal filters.
The natural fluorescence of such compositions, which has been described as an "initial dose", comprises several components. The initial dose is partially dependent on the glass composition employed, the melting conditions used, the surface finish of the glass dosimeter and chemical changes which have taken place at the surface due to atmospheric influences. Superimposed over the radio-photoluminescence and measured as apparent gamma radiation, the initial dose is generally at least 100 mRad in the most favorable case and 700-800 mRad or more in unfavorable cases, with values in most cases falling between these two extremes. Manifestly, the smaller the initial dose of such a composition, the smaller are the actual doses which can be measured accurately. Using a sensitive measuring apparatus, individual doses of about 10-20 mRad can be measured, but such accuracy is unnecessary in all cases.
After exposure to radiation, the formation of luminescence centers continues and a maximum measured indicated value is not reached for from one to several days. This procedure, known in the art as "build-up", can be accelerated by use of increased temperature; for example, heat treatment at 100.degree. C. allows final indicated values to be achieved after only 10-20 minutes. Under continued heat treatment at 100.degree. C. for longer than one hour, a slight decrease in radio-photoluminescence is noted.
The silver metaphosphate content of the glass has an an important effect on the build-up performance; in general, an increasing AgPO.sub.3 content accelerates the build-up, while a decreasing content retards build-up. However, a lower AgPO.sub.3 content may be required to reduce the effective order of magnitude, so that heat treatment before dosimeter assessment may become necessary in any case.
Silver-bearing phosphate glass dosimeters have relatively small dimensions due to their good sensitivity; for example, rod dosimeters are typically 0.1 mm.times.6 mm and plates or oblongs typically 10.times.10.times.1.5 mm. or 8.times.8.times.4.7 mm., respectively.
The dosimeter glasses generally contain large proportions of LiPO.sub.3. The lithium proportion ensures a low effective order of magnitude while at the same time giving a low natural luminescence or initial dose to the dosimeter. However, in view of its alternating effect with thermal neutrons, such lithium-containing dosimeter glass is inherently sensitive to thermal neutrons, reacting according to the nuclear process .sup.6 Li(n,.alpha.).sup.3 H. The secondary ionization caused by the resulting alpha-radiation releases the radio-photoluminescence. Thus, in the case of radiation containing gamma-radiation as well as thermal neutrons, both components are indicated as the sum of ionization.
Phosphate glasses for dosimeters with a relatively low effective order of magnitude have been described by R. Yokota et al. in Health Physics 5: 219-224 (1961), the contents of which are incorporated by reference herein. These glasses contain about 50 parts by weight LiPO.sub.3 and Al(PO.sub.3).sub.3 and are melted with the addition of approximately 2-8 parts by weight AgPO.sub.3 as an activator. These glasses are sensitive to gamma radiation and, owing to their lithium content, to thermal neutrons as well.
In most applications, it is desirable to exclude thermal neutron sensitivity so that the glasses used in such dosimeters must not contain lithium. Sodium and, in principle, potassium are available as substitutes for the alkali metal lithium in such glasses. Lithium-free silver-bearing phosphate glass dosimeters having sodium as the alkali metal component are known, e.g. see Health Physics 20: 662-663 (1971), the contents of which are incorporated by reference herein. The two dosimeter glass compositions disclosed therein contain 8.93 and 11.00 parts by weight of sodium, corresponding respectively to about 39.64 and 48.7 parts by weight of NaPO.sub.3, with the remainder of the compositions comprising aluminum and silver metaphosphate. However, sodium is not an entirely satisfactory substitute due the relatively high order of magnitude in such glasses, which contain the isotope .sup.40 K having a natural radioactivity which contributes, albeit to a limited extent, to background radio-photoluminesence.
As a result of their high alkali metal metaphosphate content, prior art silver-bearing phosphate glass dosimeters have only a limited chemical resistance and a generally poor resistance to atmospheric effects, tending to surface flaking after atmospheric exposure for a month or two and occasionally building up opaque layers at a later stage. Furthermore, in most cases the polished glass surface becomes sticky after such extended storage. While such initial surface changes can be eliminated by washing with distilled water without greatly affecting the initial dose, more marked effects result in irreversible damage, so that the practical application of such dosimeter glasses is limited.
In dosimeter glasses having a relatively low effective order of magnitude, relatively low initial doses (less than about 1 Rad) and high sensitivity, it has heretofore been assumed that a high alkaline metaphosphate content, e.g. of 30-50% by weight, is required. However, as previously noted, such high alkaline metaphosphate contents result in poor resistance to atmospheric weathering. In principle, it is possible to improve the surface chemical resistance by the addition of beryllium oxide or beryllium phosphate, as the low position of beryllium in the periodic table of the elements is an advantage in favoring low energy dependence. However, this is not a desirable alternative in view of the extensive safety measures required during melting and processing to avoid potential danger to workers and the enviornment casused by the extreme toxicity of beryllium compounds.
Alternatively, the use of magnesium oxide or magnesium metaphosphate can be used to improve the surface chemical resistance, e.g. a sodium aluminum phosphate glass containing magnesium phosphate as a silver-activated dosimeter glass is described in Toshiba's Catalog No. 4211, "Fluoro-Glass Dosimeter" containing 29.27% NaPO.sub.3 ; 14.63% Mg(PO.sub.3).sub.2 ; 53.66% Al(PO.sub.3).sub.3 ; and 2.44% AgPO.sub.3. However, the sensitivity of this glass is somewhat less than that of a comparable magnesium-free dosimeter glass and the initial dose increases as the result of the changed chemical composition. Furthermore, this glass does not withstand long storage periods, particularly at high temperatures in humid atmospheres.